Invert primary values in a photo
You can do pretty much whatever you want with the channels of an image with the compositing nodes. Look through the nodes in the Add menu(Shift+a in the Compositor). In the Converter section there are a lot of interesting nodes and amongst them Separate RGBA and Combine RGBA nodes that split and combine channels, so you can simply connect the channels in any order you want. You can also use all kinds of maths and other manipulations: